In dentistry, practitioners use a variety of restorative materials to create crowns, veneers, direct fillings, inlays, onlays and splints. Posterior and anterior tooth restoration is typically accomplished by excavating a tooth that has decayed or is otherwise in need of repair to form a cavity. This cavity is filled with a paste material, which is then compacted and shaped to conform to the original contour of the tooth. The paste is then hardened, typically by exposure to actinic light. The paste material is a tooth colored, packable, light curable, polymerizable restorative composition comprising a highly filled material.
Tooth colored dental restorative composites are usually composed of dispersions of glass filler particles below 50 .mu.m in methacrylate-type monomer resin. Splintered pre-polymerized particles, which are ground suspensions of silica in pre-polymerized dental resins, may also be used. Additives such as pigments, initiators and stabilizers have also been used in these types of composites. Because the glass particle surface is generally hydrophilic, and because it is necessary to make it compatible with the resin for mixing, the glass filler is treated with a silane to render its surface hydrophobic. The silane-treated filler is then mixed with the resin at a proportion (load) to give a paste with a consistency considered usable, that is to allow the paste to be shaped without it flowing under its own weight during typical use. This paste is then placed on the tooth to be restored, shaped and cured to a hardened mass by chemical or photochemical initiation of polymerization. After curing, the mass has properties close to the structure of a tooth. The restorative composites may be dispersion reinforced, particulate reinforced, or hybrid composites.
Dispersion reinforced composites include a reinforcing filler of, for example, fumed silica having a mean particle size of about 0.05 .mu.m or less, with a filler loading of about 30%-45% by volume. Because of the small particle size and high surface area of the filler, the filler loading into the resin is limited by the ability of the resin to wet the filler. Consequently, the filler loading is limited to about 45% by volume. Due to the low loading, the filler particles are not substantially in contact with one another. Thus, the primary reinforcing mechanism of such dispersion reinforced composites is by dislocation of flaws in the matrix around the filler. In dispersion reinforced materials, the strength of the resin matrix contributes significantly to the total strength of the composite. In dentistry, dispersion reinforced composite resins or microfills are typically used for cosmetic restorations due to their ability to retain surface luster. Typically, these microfill resins use free radical-polymerizable resins such as methacrylate monomers, which, after polymerization, are much weaker than the dispersed filler. Despite the dispersion reinforcement, microfill resins are structurally weak, limiting their use to low stress restorations.
One example of a dispersion reinforced composite is HELIOMOLAR.RTM., which is a dental composite including fumed silica particles on the order of 0.05 .mu.m mean particle size and rare earth fluoride particle on the order of less than 0.2 .mu.m mean particle size. HELIOMOLAR.RTM. is a iadiopaque microfill-type composite. The rare earth fluoride particles contribute to both flexural strength and radiopacity.
Particulate reinforced composites typically include a reinforcing filler having an average particle size greater than about 0.6 .mu.m and a filler loading of about 60% by volume. At these high filler loadings, the filler particles begin to contact one another and contribute substantially to the reinforcing mechanism due to the interaction of the particles with one another and to interruption of flaws by the particles themselves. These particulate reinforced composite resins are stronger than microfill resins. As with the dispersion reinforced composites, the resin matrix typically includes methacrylate monomers. However, the filler in particulate reinforced composites has a greater impact on the total strength of the composite. Therefore, particulate reinforced composites are typically used for stress bearing restorations.
Another class of dental composites, known as hybrid composites, include the features and advantages of dispersion reinforcement and those of particulate reinforcement. Hybrid composite resins contain fillers having an average particle size of 0.6 .mu.m or greater with a microfiller having an average particle size of about 0.05 .mu.m or less. HERCULTLTE.RTM.XRV (Kerr Corp.) is one such example. HERCULITE.RTM. is considered by many as an industry standard for hybrid composites. It has an average particle size of 0.84 .mu.m and a filler loading of 57.5% by volume. The filler is produced by a wet milling process that produces fine particles that are substantially contaminant free. About 10% of this filler exceeds 1.50 .mu.m in average particle size. In clinical use, the surface of HERCULITE.RTM. turns to a semi-glossy matte finish over time. Because of this, the restoration may become distinguishable from normal tooth structure when dry, which is not desirable for a cosmetic restoration.
Another class of composites, flowable composites, have a volume fraction of structural filler of about 10% to about 30% by volume. These flowable composites are mainly used in low viscosity applications to obtain good adaptation and to prevent the formation of gaps during the filling of a cavity.
In U.S. Pat. No. 6,121,344 filed Mar. 17, 1999 and entitled "Optimum Particle Sized Hybrid Composite", which is incorporated by reference herein in its entirety, it was found that resin-containing dental composites that incorporate a main structural filler of ground particles of average particle size at or below the wavelength of light (between about 0.05 .mu.m to about 0.5 .mu.m) have the high strength required for load bearing restorations, yet maintain a glossy appearance in clinical use required for cosmetic restorations. Composites containing a main structural filler with average particle size of about 1.0 .mu.m or greater do not provide a glossy surface.
Various methods of forming submicron particles, such as precipitation or sol gel methods, are available to produce particulate reinforcing fillers for hybrid composites. However, these methods do not restrict the particle size to at or below the wavelength of light to produce a stable glossy surface. U.S. Pat. No. 5,600,67 to Noritake et al., shows an inorganic filler composition of 60%-99% by weight of spherical oxide particles having a diameter between 0.1-1.0 .mu.m, and 1%-40% by weight of oxide particles having a mean particle diameter of less than 0.1 .mu.m. This filler is manufactured by a chemical sol gel process. The particle size range includes particle sizes up to 1.0 .mu.m and thug a dental composite using such filler will not provide a glossy surface in clinical use. The particles formed by the sol-gel process are spherical as shown in FIGS. 2A and 2B. The formulations described are designed to improve mechanical performance, wear and surface roughness of restorations, but do not provide for the retention of surface gloss in clinical use. Clinical studies of this material have actually shown high wear rates of 22.4 .mu.m per year, which cannot establish a stable surface (S. Inokoshi, "Posterior Restorations: Ceramics or Composites?" in Transactions Third International Congress on Dental Materials Ed. H. Nakajima, Y. Tani JSDMD 1997).
Comminution by a milling method may also be used for forming the submicron particles. The predominant types of milling methods are dry milling and wet milling. In dry milling, air or an inert gas is used to keep particles in suspension. However, fine particles tend to agglomerate in response to van der Waals forces, which limits the capabilities of dry milling. Wet milling uses a liquid such as water or alcohol to control reagglomeration of fine particles. Therefore, wet milling is typically used for comminution of submicron-sized particles.
A wet mill typically includes spherical media that apply sufficient force to break particles that are suspended in a liquid medium. Milling devices are categorized by the method used to impart motion to the media. The motion imparted to wet ball mills includes tumbling, vibratory, planetary and agitation. While it is possible to form submicron particles with each of these types of mills, the agitation or agitator ball mill is typically most efficient.
The agitator ball mill, also known as an attrition or stirred mill, has several advantages including high energy efficiency, high solids handling, narrow size distribution of the product output, and the ability to produce homogeneous slurries. The major variables in using an agitator ball mill are agitator speed, suspension flow rate, residence time, slurry viscosity, solid size of the in-feed, milling media size and desired product size. As a general rule, agitator mills typically grind particles to a mean particle size approximately 1/1000 of the size of the milling media in the most efficient operation. To obtain mean particle sizes on the order of 0.05 .mu.m to 0.5 .mu.m, milling media having a size of less than 0.45 mm can be used. Milling media having diameters of 0.2 mm and about 0.6 mm are also available from Tosoh Ceramics, Bound Brook, N.J. Thus, to optimize milling, it is desired to use a milling media approximately 1000 times the size of the desired particle. This minimizes the time required for milling.
Previously, the use of a milling process to achieve such fine particle sizes was difficult due to contamination of the slurry by the milling media. By using yttria stabilized zirconia (YTZ or Y-TZP, where TZP is tetragonal zirconia polycrystal), the contamination by spalling from the milling media and abrasion from the mill is minimized. Y-TZP has a fine grain, high strength and a high fracture toughness. YTZ is the hardest ceramic and because of this high hardness, the YTZ will not structurally degenerate during milling. High strength Y-TZP is formed by sintering at temperatures of about 1550.degree. C. to form tetragonal grains having 1-2 .mu.m tetragonal grains mixed with 4-8 .mu.m cubic grains and high strength (1000 MPa), high fracture toughness (8.5 MPa m.sup.1/2) and excellent wear resistance. The use of Y-TZP provides a suitable milling media for providing relatively pure structural fillers having mean particle sizes less than 0.5 .mu.m.
In U.S. Pat. No. 6,010,085 filed Mar. 17, 1999 and entitled "Agitator Mill and Method of Use for Low Contamination Grinding", and U.S. Pat. No. 5,979,805 filed Dec. 4, 1998 and entitled "Vibratory Mill and Method of Use for Low Contamination Grinding", both incorporated herein by reference in their entirety, there is described an agitator mill and vibratory mill, respectively, and method of use designed to grind structural fill to a size at or below the wavelength of light with minimal contamination.
Aside from the need for achieving highly pure structural filler of particle size at or below the wavelength of light, an additional factor to be considered in developing dental composites is that the coefficient of thermal expansion of the glass fillers used in resin-based composites is much closer to tooth structure than that of the resins. So it is desirable to limit the amount of the resin in a dental composite and maximize the amount of filler material. The main factor limiting the volume fraction (load) of the inorganic filler in highly filled suspensions is particle-particle interactions. Dispersants, through their ability to reduce interactions between particles can improve the flow (reduce the viscosity) of the suspension, therefore allowing a higher load. Dispersants in non-aqueous systems are believed to reduce particle interactions by a steric stabilization mechanism. A layer of the dispersant is adsorbed on the surface of the particles keeping them apart from one another, reducing the viscosity. The dispersant structure must contain a chain that allows for steric stabilization in the resin and it also must be strongly adsorbed on the particle surface. In U.S. Pat. No. 6,127,450 filed Jun. 9, 1998, and entitled "Dental Restorative Composite", which is incorporated by reference herein in its entirety, the use of phosphate-type dispersants is described for increasing the loading in a hybrid composite in which the main structural filler has an average particle size of about 1.0 .mu.m. There is a need, however, to provide a dispersant that will be effective with a non-aqueous, highly filled suspension containing a main structural filler having a particle size at or below the wavelength of light.
In summary, the dental profession is in need of a dental restorative that has high load capabilities and high strength for load bearing restorations, yet maintains a glossy appearance in clinical use required for cosmetic restorations.